feat: add instance-based overload for create_index_on

Columns can now be passed as instances instead of template parameters:
create_index_on(index_id<"idx">{}, table{}, col1{}, col2{})

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Pierre-Luc Gagné

CHANGELOG.md

@@ -8,6 +8,10 @@ Versioning follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
 
+### Added
+
+- Instance-based overload for `create_index_on` — e.g. `create_index_on(index_id<"idx">{}, table{}, col1{}, col2{})` as an alternative to the template-only form
+
 ### Changed
 
 - `col_ref` is now instance-based — `col_ref(col{})` replaces `col_ref<col>` for consistency with the rest of the API

lib/include/ds_mysql/sql_ddl.hpp

@@ -2331,6 +2331,12 @@ template <ColumnDescriptor... Cols, fixed_string Name, ValidTable Table>
     return ddl_detail::create_index_builder<Table, Cols...>{std::string(Name)};
 }
 
+template <fixed_string Name, ValidTable Table, ColumnDescriptor... Cols>
+[[nodiscard]] ddl_detail::create_index_builder<Table, Cols...> create_index_on(index_id<Name> const&, Table const&,
+                                                                               Cols const&...) {
+    return ddl_detail::create_index_builder<Table, Cols...>{std::string(Name)};
+}
+
 template <fixed_string Name, ValidTable Table>
 [[nodiscard]] ddl_detail::drop_index_builder<Table> drop_index_on(index_id<Name> const&, Table const&) {
     return ddl_detail::drop_index_builder<Table>{std::string(Name)};

tests/unit/test_ddl.cpp

@@ -1663,6 +1663,20 @@ suite<"DDL Features"> ddl_features_suite = [] {
         expect(sql == "CREATE UNIQUE INDEX uq_test_table_name ON test_table (name);\n"s) << sql;
     };
 
+    "create_index_on instance-based - generates CREATE INDEX"_test = [] {
+        auto const sql =
+            create_index_on(index_id<"idx_test_table_id_name">{}, test_table{}, test_table::id{}, test_table::name{})
+                .build_sql();
+        expect(sql == "CREATE INDEX idx_test_table_id_name ON test_table (id, name);\n"s) << sql;
+    };
+
+    "create_index_on instance-based.unique - generates CREATE UNIQUE INDEX"_test = [] {
+        auto const sql = create_index_on(index_id<"uq_test_table_name">{}, test_table{}, test_table::name{})
+                             .unique()
+                             .build_sql();
+        expect(sql == "CREATE UNIQUE INDEX uq_test_table_name ON test_table (name);\n"s) << sql;
+    };
+
     "drop_index_on - generates DROP INDEX ON table"_test = [] {
         auto const sql = drop_index_on(index_id<"idx_test_table_id_name">{}, test_table{}).build_sql();
         expect(sql == "DROP INDEX idx_test_table_id_name ON test_table;\n"s) << sql;